A number of security systems which make use of fibre optic elements in their construction are known. In South African Pat. No. 78/5419 for example a security system is described in which fibre optic elements are located in a wall along a boundary of an area to be protected. Breakage or damage to one or more of the fibre optic elements, caused for example by an intruder, results in a reduction in the intensity of or complete loss of an optical signal in such fibre optic elements and the detection of this signal intensity reduction or loss signifies an intrusion attempt. In the specification accompanying U.K. Patent Application GB No. 2,038,060A another security system is disclosed, and in that arrangement a wall to be protected has embedded in it a mesh of optical fibres with the fibres arranged in a reticulated pattern. Each fibre has a light source at one end and light detector at the other end. If one or more of the fibres is broken then the cessation of light in such fibres is used to give an alarm.
The above known security systems tend to be both expensive and difficult to install, one needing to take great care that the fibre optical elements are not damaged during the erection of the wall in which they are being embedded. Furthermore should a fault occur in one or more of the fibre optic elements, either during the erection of subsequent to the erection of the wall, then the location of that fault and its repair may be an expensive and difficult procedure involving the destruction of a part or in extreme cases the whole of the wall. Such walls are also built so that they cannot easily be dismantled without damage to the fibre optic elements embedded within the wall. This can be a disadvantage if one wished to dismantle part of the wall to repair a fault or to dismantle the wall completely so as to re-erect it elsewhere as a major portion of the fibre optic elements may need to be replaced as a result of damage caused to them by the part or total dismantling of the wall.
U.K. Patent Application No. 2,060,966A described a security system in which a security wall is built up from a number of composite panels incorporating fibre optic elements in hollow tubes. Several panels are stacked together and mounted between a pair of box posts to define a section of the wall. The fibre optic elements in a panel in one section of the wall are connected via connectors in the box posts to fibre optic elements in a panel in an adjacent section of the wall. One of the disadvantages of this security system and the other known security systems described above is that they provide only a limited degree of security in that an intruder once having carefully removed part of the wall and gained access to the fibre optic elements may then succeed in gently stretching adjacent fibre optic elements apart so as to create a gap sufficient to squeeze through, without there being any rupture of the optic fibre elements. It is possible to stretch fibre optic elements apart in this way because of the elasticity of the fibres some of which can be extended by up to 3% before they rupture.
In the security wall described in U.K. Patent Application No. 2,060,966A an intruder may also attempt to gain access by releasing a panel from the box posts and from its adjacent panel or panels in the same section of the wall, the fibre optic elements within the panel still having their ends connected to the connectors in the box posts. Then in view of the long lengths of fibre optic elements within the hollow tubes those lengths of fibre optic elements may be sufficiently extensible to allow sufficient rotation of that individual panel so as to gain access through the wall.